The invention relates to the field of oversampled pipeline A?D converters with mismatch shaping.
In recent years, delta-sigma modulators have become very popular as analog-to-digital conversion circuits. The advantages of delta-sigma A/D converters compared with conventional A/D converters are: 1) shaping of quantization noise thereby reducing in-band quantization noise; 2) ease of anti-aliasing due to inherent oversampling; 3) tolerance to component mismatches.
In delta-sigma converters employing a single loop and 1-bit feedback, the converter is inherently linear, and hence does not require good component matching. However, such converters generally require a large oversampling ratio in order to achieve a high signal-to-noise ratio. The reason for this is, first, the quantization noise of the resulting converter is directly proportional to the quantization noise of the D/A converter in the feedback path. The 1-bit D/A converter in the feedback introduces large quantization noise. Second, the order of the loop has a diminishing effect on the signal-to-noise ratio for the loop to remain stable at high order.
Cascaded or MASH structure was developed to circumvent the instability of high-order delta-sigma converters, thus increasing the signal-to-noise ratio at a given oversampling ratio. Unfortunately, the MASH is much less tolerant to component mismatches, and moreover, is prone to limit-cycle tones. This makes MASH unsuitable for applications where low oversampling ratio is desired. Another attempt for increasing the signal-to-noise ratio of delta-sigma converters is to increase the number of bits in the feedback D/A converter. An unfortunate consequence of multi-bit feedback is that the D/A converter must now be accurate to the final resolution of the delta-sigma converter. For example, in a delta-sigma converter with a target SNDR of 98 dB (16 bits), the D/A converter in the feedback path must be accurate to 16 bits despite the fact that the D/A converter may have only 2 bit resolution. A number of techniques have been developed to overcome the accuracy problems in the multi-bit feedback A/D converters. They include self-calibration, dynamic component matching, and mismatch shaping.
On the contrary, pipeline converters have been traditionally applied to Nyquist-rate sampling. Although pipeline A/D converters are relatively simple and power efficient, component mismatches limit the accuracy to about 10 bits. Numerous techniques that include self-calibration, error averaging, ratio-independent, and reference refreshing methods have been devised to remove errors due to component mismatches. A technique applicable only to cyclic A/D converter has also been devised. Except for self-calibration techniques, all techniques take up extra cycles of the valuable conversion time, reducing the throughput of the converter significantly, and the added complexity often increase the power consumption considerably.
Self-calibration techniques, on the other hand, remove component mismatches without increasing conversion time. Depending on a particular self-calibration technique, power consumption can be comparable to standard pipeline converters or can be substantially increased. The drawback of self-calibration is the additional complexity and the necessity of the calibration period during which the converter is inoperable or has lower accuracy. A recently reported pipeline converter using commutative feedback capacitors does not require self-calibration, neither increases conversion time nor adds more power consumption. Although this technique offers superior differential linearity, the integral linearity still depends on capacitor matching. Therefore, this technique is suitable for applications in which the integral linearity is less important than differential linearity.
In comparing delta-sigma converters and pipeline converters for wide-band signals, for example, 50-100 MHz IF for spread-spectrum receivers, a few important attributes are recognized. Due to the wide bandwidth of the input signal and limited circuit speed, delta-sigma converters afford only low oversampling ratios, which makes high-resolution conversion extremely difficult. The low oversampling ratio generally nullifies the primary advantage of delta-sigma converters; the tolerance to component mismatches.
In this regard, remaining advantages of delta-sigma converters over pipeline converters now only include ease of anti-alias filtering and low quantization noise. It must be noted that the ease of anti-aliasing is not inherent to delta-sigma modulation. Rather, it is associated with oversampling. Therefore, pipeline converters can experience the same benefit of easy anti-aliasing simply operating the converter at higher sampling rate than the Nyquist rate, i.e., oversampling. As for quantization noise in pipeline converters, the quantization noise can be made smaller by adding more stages at the end of the pipeline. Since the last stages of the pipeline do not contribute much thermal noise, they can be made extremely small and low power. Therefore, the quantization noise itself can be made arbitrarily small with negligible increase of area and power. Certainly, doing so will not improve the accuracy or thermal noise. However, it is no different in delta-sigma converters with low oversampling ratio.
Based on the above observation, it can be concluded that delta-sigma converters do not possess any fundamental advantage over pipeline converters for wide band applications that necessitates low oversampling ratios. As a practical matter, at the input frequencies over 50 MHz, it would be extremely difficult to achieve oversampling ratio greater than 8 with final accuracy over 12 bits with present technologies. At this low oversampling ratio, not only the benefits of delta-sigma modulation disappears, but more importantly many delta-sigma converters are incapable of providing good enough performance. One exception is presented where a 12-bit pipeline A/D converter is employed in the back-end of a delta-sigma converter with truncated feedback. This converter reportedly achieved 16 bit SNDR with the oversampling ratio of 8. A possibly more efficient approach would be to oversample a standard pipeline converter, and shape the mismatch out of band which will be removed by a subsequent digital filter. Since no attempt is made to shape the quantization noise, there is none of the concerns associated with delta-sigma converters with a low oversampling ratio.